Image projection systems which use reflective modulation fields as image transmitters and image onto projection screens or projection walls are known.
This type of projection system is described among other places in WO 2005/045507 A1. It comprises a Liquid Crystal on Silicon (LCOS) element, a projection lens arrangement and a wire grid in order to direct at least part of the light from the source to the LCOS element and to direct part of the light reflected by the LCOS element onto the projection lens arrangement. Moreover, a field lens and a quarter-wave plate are disposed optically between the LCOS element and the wire grid. By means of the quarter-wave plate between the LCOS element and the wire grid the contrast is improved by correcting polarization errors which otherwise occur due to the lack of telecentricity of the field lens.
Systems of this type generally comprise many lens elements and are very complex in structure and so expensive to produce.
It is the object of the invention to provide a projection objective which with a small number of optical components and the smallest dimensions guarantees a high image quality and inexpensive production.
This object is achieved by the projection system according to claim 1 and the projector according to Claim 12. Preferred embodiments of the invention form the subject matter of the respective sub-claims.
The idea forming the basis of the invention is to construct the projection objective, starting from the side facing towards the projection wall, from a first group (G1) which comprises a negative meniscus, a second group (G2) which comprises a positive kit group or a positive lens, a third group (G3) which comprises a single lens with positive or negative refractive power, and a fourth group (G4) which comprises a positive lens and a field lens (FL) with positive refractive power.
The projection objective according to the invention for projecting light onto a projection surface that comprises: a first group (G1) with a negative meniscus, a second group (G2) with positive refractive power, a third group (G3) with a single lens, a fourth group (G4) with a lens with positive refractive power and a field lens (FL) with positive refractive power, the light passing through the projection objective from the fourth to the first group, is characterized in that the common focal width of the fourth group (G4) and the field lens (FL) is smaller than twice the image circle diameter of the projection objective, and the maximum free diameter is smaller than the overall focal width and in particular smaller than 0.8 times the overall focal width of the projection objective.
In preferred embodiments the projection objective according to the invention has as one or a number of further features that:                the second group comprises a kit group;        the second group comprises a single lens;        all of the lenses are spherical;        the field lens is made of a material with a low stress birefringence, in particular Pockelt glass;        the focal width of the field lens is greater than three times the image circle diameter of the projection objective;        the focal width of the fourth group is smaller than three times the image circle diameter of the projection objective;        the common focal width of the fourth group and the field lens is smaller than the overall focal width of the projection objective;        the maximum free diameter is smaller than one and a half times the image circle diameter of the projection objective;        the overall focal width of the projection objective is smaller than one and a half times the distance between the fourth group and the field lens; and        the distance between the fourth group and the field lens is substantially as great as the distance between the first group and the fourth group.        
Correspondingly, the projector according to the invention for producing an image on a projection surface with an image transmitter for modulating light and a projection objective for projecting the modulated light onto the projection surface is characterized in that the projection objective is designed in the manner of one of the above embodiments.
In preferred embodiments the projector according to the invention has as one or a number of further features that:                the image transmitter comprises LED illumination with a number of color channels, each color channel having at least one LED;        each LED has a focusing element for reducing the angle range in which light is radiated by the LED;        the focusing element comprises a lens system and/or a parabolic collection element;        a color channel consolidation unit for consolidating the color channels behind the focusing element;        the color channel consolidation unit comprises an x plate or an x cube;        an illumination homogenization element for homogenizing the illumination of the image transmitter is provided;        the illumination homogenization element is in the form of a honeycomb condenser;        the illumination homogenization element is in the form of a parabolic collection element; and        the image transmitter functions dependently upon polarization and a polarizer is disposed in front of the field lens.        
The advantage of the projection objective according to the invention is that with it one can construct a compact projector and moreover the illumination of the image transmitter can also be compact in form. If LEDs are used as light sources, it is a further advantage that the latter have a very long life and are very efficient. A projection objective with a fixed focal width is thus provided which is suitable in particular for imaging reflective image modulation fields.